1. Field of the Invention
This invention relates generally to magneto-resistive sensor devices for detecting a magnetic field impinging upon the magneto-resistive sensor device. More particularly, this invention relates to magnetic field angle sensors for measurement of a magnetic field angle over a 360° range of measurement.
2. Description of Related Art
Magnetic position sensing is becoming a popular method of implementing a non-contacting location of objects in motion. By affixing a magnet or sensor element to an angular or linear moving object with its complementary sensor or magnet stationary, the relative direction of the resulting magnetic field can be quantified electronically. By utilizing multiple sensors or magnets, the capability of extended angular or linear position measurements can be enhanced. “Applications of Magnetic Position Sensors”, Honeywell Application Note-AN211, found www.ssec.honeywell.com/magnetic/datasheets/an211.pdf, Mar. 20, 2007 describes magnetic position sensing using Anisotropic Magneto-Resistive (AMR) sensors.
Further, AN211 describes the use of an anisotropic magneto-resistive material such as Permalloy to form four anisotropic magneto-resistive (AMR) elements 10a, 10b, 10c, and 10d that are connected as a Wheatstone bridge sensor 5, as shown in FIG. 1. Each magneto-resistive element 10a, 10b, 10c, and 10d possesses an ability to change resistance in a cos2(θ) relationship where θ (theta) is the angle between the magnetic moment (M) vector 25a, 25d, 25c, and 25d and the current flow (I) 20a, 20d, 20c, and 20d. 
The sensor is formed from the AMR elements 10a, 10b, 10c, and 10d, the four elements 10a, 10b, 10c, and 10d are oriented in a diamond shape with the ends connected together by metallization 12a, 12b, 12c, and 12d to form the Wheatstone bridge. The top and bottom connections of the four identical elements are connected Direct Current (DC) power supply voltage source (Vs) 15. The remaining two opposing side connection terminals 12c and 12d are the sense point of the measurement. With no magnetic field supplied (0 gauss), the side connection terminals 12c and 12d have an equal voltage level with the exception of a small offset voltage due to manufacturing tolerances on the AMR elements 10a, 10b, 10c, and 10d. The Wheatstone bridge connection structure 5 produces a differential voltage (ΔV) as a function of the supply voltage Vs, The ratio of the resistance of the AMR elements 10a, 10b, 10c, and 10d, and the angle (θ) between the element current flow (I) 20a, 20d, 20c, and 20d and element magnetization (M) 25a, 25d, 25c, and 25d 
The Wheatstone bridge 5 as constructed provide an angle measurement of +/−45°. To provide measurement of from +/−45° to +/−90° requires two Wheatstone sensors with 45° displacement from each other, the two linear slopes can be used additively. A full 360° rotational position sensing solution uses two of the Wheatstone bridge sensors 5 combined with a hall effect sensor. Most hall effect sensors use silicon semiconducting materials to create a proportional voltage output as a magnetic field vector slices orthogonally through the slab material with a bias current flowing through it to generate a signed vector of the impinging magnetic field.
“Angular Sensor Using Tunneling Magneto-resistive Junctions with an Artificial Antiferromagnet Reference Electrode and Improved Thermal Stability”, Ruhrig, et al., IEEE Transactions on Magnetics, January 2004 Volume: 40, Issue: 1, pp.: 101-104, describes fabrication of Magnetic tunneling junctions (MTJs) using CoFe—Ru—CoFe artificial antiferromagnet (AAF) sandwiches as a hard-magnetic reference layer and plasma-oxidized aluminum as a tunnel barrier. Tailoring the magnetic properties of the artificial antiferromagnet reference layer allows an on-chip magnetization (initialization) of individual junctions, which makes it possible to build monolithic Wheatstone without multiple mask process steps or on-chip heating elements. It should be noted that the MTJ elements of Ruhrig et al. require that each MTJ element be initialized individually with locally applied current pulses. This is not practical for mass production applications.
“360° Angle Sensor Using Spin Valve Materials with SAF Structure”, Wang et al., IEEE Transactions on Magnetics, October 2005, Volume: 41, Issue: 10, pp.: 3700-3702, illustrates the design, fabrication and test of microchips of 360° angle sensors using spin valve materials. The angle sensor is used stationary in combination with a disc-shaped permanent magnet attached to a rotating shaft near the sensor. The permanent magnet is magnetized in-plane, thus creating a field that is rotating with the shaft. The magnetic field from the permanent magnet forces the free layer magnetization to follow the field and rotate with it. With a fixed reference layer magnetization and an in-phase following of the free layer magnetization, the magneto-resistance is a simple cosine function of the angle between the rotating permanent magnet and the stationary sensor. A special Wheatstone-bridge with four separate spin valve resistors is used to compensate the thermal drift expected in application environments. One half bridge has a 90° phase delay from the other, resulting in a cosine and a sine function, in combination to uniquely determine any angular relationship between the permanent magnet and the sensor between 0° to 360°. A draw back of the 360° Angle Sensor of Wang et al. is that it is not a single chip integrated solution, but a multiple component implementation that is costly and may introduce errors.
European Patent EP0910802 (Lenssen) provides a magnetic field sensor comprising: resistive elements in a Wheatstone bridge configuration. At least one element is a magneto-resistive device. A measurement current is passed from a first point through the bridge to a second point. A conductive track runs in proximity to but is electrically insulated from the resistive elements for the magnetically biasing the resistive elements with a biasing current. The second point is electrically connected to the conductive track so that the measurement current is also employed as the biasing current.
European Patent EP0760931 (Andrae, et al.) describes a sensor for sensing at least one of angular position and rotation speed. The sensor includes a permanent magnet rotatable about an axis of rotation and at least three Wheatstone bridges each having four bridge resistors formed of magneto-resistive strip lines extending in planes parallel to a rotation plane of the permanent magnet. The bridge resistors of each of the Wheatstone bridges are disposed on respective sides of quadrangles corresponding to the respective Wheatstone bridges. Each Wheatstone bridge has two adjacently arranged bridge resistors connected to a half-bridge of the Wheatstone bridges. The quadrangles are disposed relative to one another rotated by a preselectable angle. The three Wheatstone bridges each have an intersection point of virtual diagonals connecting corners of respective ones of the quadrangles, the intersection points being arranged substantially concentrically about the axis of rotation, and the three Wheatstone bridges are disposed so as to be equally and evenly swept by a field of the permanent magnet during rotation thereof to saturate the bridge resistors.
European Patent EP1481256 (Wan, et al.) provides an integrated magnetic field sensing device that includes at least two magneto-resistive elements. The magneto-resistive elements are biased in one direction by an integral conductor and are sensitive to magnetic field components in a direction perpendicular to the one direction. The sensitivity of the device to a magnetic field is adjustable and is related to the level of the bias current. In a current measuring application, two of the magnetic field sensing devices are mounted on opposite sides of and perpendicular to a conductor carrying a current to be measured. In a portable current measuring apparatus, two of the magnetic field sensors are mounted in a housing that assists in locating the magnetic field sensors relative to the conductor carrying the current to be measured.
U.S. Pat. No. 5,764,567 (Parkin) describes a magnetic tunnel junction (MTJ) device, an external magnetic field sensor. The MTJ device has a tunneling magneto-resistance response, as a function of applied magnetic field, that is substantially symmetric about zero field. The magnetic tunnel junction is made up of two ferromagnetic layers, one of which has its magnetic moment fixed and the other of which has its magnetic moment free to rotate, an insulating tunnel barrier layer between the ferromagnetic layers for permitting tunneling current perpendicularly through the layers, and a nonferromagnetic layer located at the interface between the tunnel barrier layer and one of the ferromagnetic layers. The nonferromagnetic layer increases the spacing between the tunnel barrier layer and the ferromagnetic layer at the interface and thus reduces the magnetic coupling between the fixed and free ferromagnetic layers, which has been determined to be the cause of unsymmetric tunneling magneto-resistance response about zero field. Even though the nonferromagnetic interface layer presents nonspin-polarized electronic states at the tunnel barrier layer interface, it unexpectedly does not cause a suppression of the tunneling magneto-resistance.
U.S. Pat. No. 6,011,390 (Loreit, et al.) describes a sensor chip with magneto-resistive Wheatstone bridges for determining magnetic field directions. The arrangement for a magneto-resistive sensor chip has two Wheatstone bridges to determine the sine and cosine of the angle formed between a chip edge and the direction of the magnetic field. All resistances of the bridges consist of a plurality of magneto-resistive laminated elements with current connections made of highly conductive thin films with parallel edges. When the resistances of a bridge are directly electrically interconnected, these edges form angles of 90° each. The parallel edges of the corresponding resistances of the sine and cosine bridges are mutually offset by 45°. The magneto-resistive laminated elements are distributed on the chip surface to reduce angle measurement errors to a minimum.
U.S. Pat. No. 6,100,686 (Van Delden, et al.) illustrates a magnetic field sensor with double Wheatstone bridge having magneto-resistive elements. The magnetic field sensor has a substrate on which a plurality of resistive elements form a double Wheatstone bridge circuit, at least one of the resistive elements in each bridge having a magneto-resistive characteristic. The two bridges are identical except in that, if a given magneto-resistive element in a given branch in one bridge has a positive output polarity, then the corresponding magneto-resistive element in the same branch in the other bridge will have a negative output polarity. By adding the output signals of the two Wheatstone bridges a zero-point offset of the sensor can be determined and eliminated. There is no need to employ the so-called flipping technique employed for that purpose in conventional sensors, which requires increased power consumption.
U.S. Pat. No. 6,633,462 (Adelerhof) teaches a magneto-resistive angle sensor which determines a magnetic field direction with a high angular accuracy over a wide range of magnetic field strengths. The magneto-resistive angular sensor includes a main sensing element which is electrically connected to a two correction sensing elements. The first correction sensing element has a first reference magnetization axis and the second correction sensing element has a second reference magnetization axis. The first and the second reference magnetization axes make correction angles θ between 5° and 85° of opposite sign with a main reference axis.
U.S. Pat. No. 6,756,782 (Van Zon) describes a magnetic field measuring sensor having a shunt resistor and method of regulating the sensor. The sensor for measuring a magnetic field includes a substrate, four magnetic elements arranged in a bridge configuration on the substrate, a first bridge portion. The two elements are connected in series, a second bridge portion, and a third element and a fourth element are connected in series, being situated between a first contact and a second contact. The first bridge portion includes an electrical shunt resistor, which is arranged parallel to the first magnetic element of the bridge. In order to compensate for offset voltage and offset voltage drift in the output voltage of the bridge configuration, the temperature coefficient of the shunt resistor compensates for the variation of the temperature coefficients of the magnetic elements in the bridge.
U.S. Pat. No. 6,771,472 (Mao, et al.) provides a magnetic sensor having a first opposing pair and a second opposing pair of resistive elements configured in a Wheatstone bridge, wherein the resistive elements are a synthetic antiferromagnetic giant magneto-resistive sensor having a reference layer and a pinned layer of different thicknesses. The first opposing pair has a net magnetic moment that is opposite to that of the second opposing pair, and the first opposing pair has a thicker reference layer than pinned layer, and the second opposing pair has a thicker pinned layer than reference layer.
U.S. Pat. No. 6,891,368 (Kawano, et al.) describes a magneto-resistive sensor device formed on a substrate with a sensing portion and a signal processing circuit. The sensing portion detects changes in a magnetic field induced by a moving body, is located at a position for effectively detecting changes in a magnetic field induced by the moving body, and is constituted by a magneto-resistive sensor element having a Wheatstone bridge configuration.
U.S. Pat. No. 6,992,869 (Suzuki, et al.) illustrates a magnetic resistance device having a high magnetic resistance change rate, satisfactory production yield and a low level of variation in production. The device has a pair of magnetic tunnel resistance devices employing a laminated structure comprised of antiferromagnetic film, lower magnetic layer, barrier film and upper magnetic layer. The pair of magnetic tunnel resistance devices are formed connected in series on substrate.
U.S. Pat. No. 7,054,114 (Jander, et al.) provides a ferromagnetic thin-film based magnetic field sensor with first and second sensitive direction sensing structures. The direction sensing structures each have a nonmagnetic intermediate layer with two major surfaces on opposite sides thereof having a magnetization reference layer on one and an anisotropic ferromagnetic material sensing layer on the other. The direction sensing structures have a length and a smaller width. The width is placed parallel to the relatively fixed magnetization direction. The relatively fixed magnetization direction of the magnetization reference layer in the direction sensing structures is oriented substantially parallel to the substrate but substantially perpendicular to that of the other direction sensing structures. An annealing process is used to form the desired magnetization directions.